In recent years, reduction of emissions of carbon dioxide and its effective utilization have been urgent issues because carbon dioxide is a main contributor to global warming. Hydrocarbon-based gases are generated from technical fields such as petroleum refining and petroleum chemistry, and a method for efficiently converting such gases into effective substances has been required.
Under this circumstance, there has been proposed a method in which a reverse shift reaction is carried out using hydrogen and carbon dioxide to produce a synthesis gas comprising generated carbon monoxide and unreacted hydrogen (see Patent Documents 1 and 2).
As shown in BACKGROUND ART (paragraph [0002]) of Patent Document 1, a method is known in which carbon dioxide in a gas after reforming water vapor is separated and fed back to a reformer.
Many catalysts have been put to use concerning catalysts for promoting a shift reaction represented by Formula (1) described below, i.e., catalysts for shift reaction, for the purpose of producing hydrogen.CO+H2O→CO2+H2  (1)
Many of the catalysts for promoting a shift reaction are considered to have an activity as a catalyst for the reverse shift reaction represented by Formula (2) described below.CO2+H2→CO+H2O  (2)
It is desirable that the reverse shift reaction be carried out at a high temperature of 600° C. or higher in consideration of the equilibrium composition of the synthesis gas generated by the reaction. However, since the temperature of 600° C. or higher is much higher than the temperature for carrying out a shift reaction in general, it is actually difficult to use these common catalysts for shift reaction.
A reverse shift reaction can proceed in a common water vapor reforming catalyst, but there is the problem that under pressurized conditions, a methanation which is described by Formula (3) below, occurs. This is a reaction reverse to the water vapor reforming reaction, and the methane generated reduces the concentration of carbon monoxide.CO+3H2→CH4+H2O  (3)